1. Field of Invention
The present invention relates to a high speed data transmission area, and more particularly to a current segmentation circuit for optimizing output waveform from high speed data transmission interface that can depress overshoot and eliminate turning point in the waveform.
2. Description of Related Arts
In data transmission area, with technology development, the conventional transmission rate can not meet the current demand. The high data transmission rate up to hundreds of Mb/s and even to a few Gb/s has been widely used.
High speed data transfer challenges the reliability of data transmission. The conventional high speed data transmission has two main problems: overshoot and the difficulty to control rising and falling time. The conventional current segmentation circuit can solve these two problems, but also bring a new one, which is the turning point in the waveform which will affect the reliability of data recovery.
The conventional high speed data transmission is based on the RC charge-discharge principle, which is to control the rising and falling time of high speed data by charging and discharging the “gate” of the switches of the differential data paths via a RC circuit. However, this technique will cause overshoot in the rising and falling edges and it is difficult to control the rising and falling time.
Referring to FIG. 1 of the drawings, a schematic view of a conventional RC charge-discharge principle is illustrated. Connect the input signal through a resistance to the “gate” of the MOS transistor switches (S1 and S2). The input falling time is controlled by the RC discharging process at the “gate”, and the input rising time is controlled by the RC charging process. Due to the exponential characteristic of the RC circuit, the changing of the “gate” voltage is divided into a fast region and a slow region. In the fast region, due to the large current, the data line (output) is charged to a relatively high value very quickly, and thus during the slow process, the switch will further raise the voltage of the data line to allow the control signal to change, which is called the overshoot. Moreover, Due to the exponential characteristic of the RC circuit, it is also difficult to control the rising and falling time in the waveform.
Therefore, in order to solve the above-mentioned problems, a current segmentation circuit is introduced. The principle of the current segmentation circuit is illustrated as below. Charge the output data with small current during the fast region, and charge the output data with large current during the slow region, so as to depress the overshoot and easily control the rising and falling time. However, due to the discontinuity of the current, the turning point during the data rising and falling process will greatly affect the reliability of data recovery.
Referring to FIG. 5 of the drawings, the output of the conventional RC controlled high speed data transmission and an eye pattern thereof is illustrated. V (S1) and V (S2) are the “gate” voltage of the MOS transistors, wherein the exponential catachrestic of the RC circuit can be seen. V (dp)−V (dm) is the differential data of the high speed transmission, wherein the overshoot and relatively short rising and falling time are obvious, and even the R or C value is greatly changed, the rising and falling time still changes very little.
Referring to FIG. 6 of the drawings, the output of the current segmentation circuit controlled high speed data transmission and an eye pattern thereof is illustrated. V (K1), V (K2), V (K3) and V (K4) are the “gate” signal of four PMOS transistors. V (dp)−V (dm) is the differential data of the high speed transmission, wherein the overshoot is greatly depressed, but the there is obvious turning point during the rising and falling process.
Take the rising process of the differential data as an example, the existing of the turning point is explained as follows. When V (K1) is start to fall, V (K1) changes quickly. During this period, the exponentially increasing current I1 flow into DP line and the voltage of the differential data rises very quickly. During the period of V (K1) falling to a relatively low voltage, the current I2 controlled by V (K2) has not flow into DP line yet, so that the exponential characteristic of the RC circuit is obvious, that is to say the charging current to the DP line increases slowly, and the differential voltage also increases very slow. During the falling process of V (K2), the extra exponentially increasing current flows into DP line from I2, so as to depress the influence of the remaining exponential current I1. From the above explanation, it can be seen that the current segmentation circuit can depress the overshoot, but will inevitably produce turning point.
So far, there is no efficient high speed data transmission method that can depress the overshoot and do not produce turning point.